STUDY OF EXTREME-ULTRAVIOLET EMISSION AND PROPERTIES OF A CORONAL STREAMER FROMPROBA2/SWAP,HINODE/EIS AND MAUNA LOA Mk4 OBSERVATIONS

Wide-field extreme-ultraviolet (EUV) telescopes imaging in spectral bands sensitive to 1 MK plasma on the Sun often observe extended, ray-like coronal structures stretching radially from active regions to distances of 1.5–2 R ☉ , which represent the EUV counterparts of white-light streamers. To explain this phenomenon, we investigated the properties of a streamer observed on 2010 October 20 and 21, by the PROBA2 /SWAP EUV telescope together with the Hinode /EIS (HOP 165) and the Mauna Loa Mk4 white-light coronagraph. In the SWAP 174 Å band comprising the Fe ix –Fe xi lines, the streamer was detected to a distance of 2 R ☉ . We assume that the EUV emission is dominated by collisional excitation and resonant scattering of monochromatic radiation coming from the underlying corona. Below 1.2 R ☉ , the plasma density and temperature were derived from the Hinode /EIS data by a line-ratio method. Plasma conditions in the streamer and in the background corona above 1.2 R ☉ from the disk center were determined by forward-modeling the emission that best fit the observational data in both EUV and white light. It was found that the plasma in the streamer above 1.2 R ☉ is nearly isothermal, with a temperature of T = 1.43 ± 0.08 MK. The hydrostatic scale-height temperature determined from the evaluated density distribution was significantly higher (1.72 ± 0.08 MK), which suggests the existence of outward plasma flow along the streamer. We conclude that, inside the streamer, collisional excitation provided more than 90% of the observed EUV emission, whereas, in the background corona, the contribution of resonance scattering became comparable with that of collisions at R ≳ 2 R ☉ .